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396                         MBTALLUEGY OE1 IROH AND STEEL.
understanding. If the tests aie made on the center portion the allowable maximum of phosphorus and sulphur should be raised 50 per cent; e. g., from .04 to .06 or .06 to .09 per cent.
The elements other than phosphorus need not be rigidly limited, for some discretion should be left to the maker in the attainment of definite physical results. It is not uncommon to find specifications that give an upper limit for'every element and require a tensile strength which cannot be obtained by the formula. The carbon should always be left open, so that if the maker wishes to reduce the phosphorus he may use carbon to get strength. Manganese may be limited to .60 per cent, on the steels under 64,000 pounds per square inch, and to .80 per cent, on harder metal. This will ensure a safe material, and not be a burden on the manufacturer. Silicon is of little importance, but the maximum may be placed at .04 per cent, for soft steel.
Sulphur concerns the manufacturer more than the engineer, for if too high the bar will crack in rolling and be imperfect, while it has no marked effect on the ductility of the finished piece. In eye-bars, however, there is danger.that high sulphur may cause crystallization during the heating necessary to form the eye.
Copper may be entirely neglected, for no ill effect upon the cold properties of low steel has ever been traced to its action, while thousands of tons of excellent metal have been made with a content of .75 per cent.
Rivet steel, like eye-bar flats, stands on a different footing from other structural metal, for this must be heated and worked after leaving the place of manufacture. Only the best of material should be used, and it should be so soft that it will not be injured by cold working or crystallized by overheating. The phosphorus should not be over .04 per cent., the sulphur not over .05 per cent., and the tensile strength not ovep 60,000 pounds per square inch.
SEO. XVIIIc.—Use of soft steel in structural worlc.—It. is not ipossible^Afl^rarily state just what is the best tensile strength for every purpose, blatHi my opinion a softer metal should be used for irises than is often 'employed, because, although a slight sacrifice ismhte ita. the ultimate strength, there is a gain in working strength due.'to nigher elastic rsttio^ anKi a- decided increase in toughness and iresisttti^ to shock, so that the calculations may be made on the sarnie b&lll fdi the working load as with a harder metal. The fact